Merkel cells ubiquitously distribute in the skin of vertebrates, from cyclostomes to mammals. It is well known that mammalian Merkel cells coupled with axon terminals of type I sensory nerve fibers form slowly adapting mechanoreceptors, Merkel endings, within the epidermis. However, there are still many unresolved problems in the biology of Merkel cells. We reviewed recently acquired knowledge about the histochemical nature of Merkel cell granules, the morphological heterogeneity of Merkel cells and the roles of neurotrophins and their receptors for the development and survival of the cells. We discuss the functional significance of Merkel cell granules and the heterogeneity of Merkel cell populations.
It is widely accepted that Meckel's cartilage in mammals is uncalcified hyaline cartilage that is resorbed and is not involved in bone formation of the mandible. We examined the spatial and temporal characteristics of matrix calcification in Meckel's cartilage, using histochemical and immunocytochemical methods, electron microscopy and an electron probe microanalyser. The intramandibular portion of Meckel's cartilage could be divided schematically into anterior and posterior portions with respect to the site of initiation of ossification beneath the mental foramen. Calcification of the matrix occurred in areas in which alkaline phosphatase activity could be detected by light and electron microscopy and by immunohistochemical staining. The expression of type X collagen was restricted to the hypertrophic cells of intramandibular Meckel's cartilage, and staining with alizarin red and von Kossa stain revealed that calcification progressed in both posterior and anterior directions from the primary centre of ossification. After the active cellular resorption of calcified cartilage matrix, new osseous islands were formed by trabecular bone that intruded from the perichondrial bone collar. Evidence of such formation of bone was supported by results of double immunofluorescence staining specific for type I and type II collagens, in addition to results of immunostaining for osteopontin. Calcification of the posterior portion resembled that in the anterior portion of intramandibular Meckel's cartilage, and our findings indicate that the posterior portion also contributes to the bone formation of the mandible by an endochondral-type mechanism of calcification.
Our pervious electron microscopic studies indicated that Merkel cells (MCs) in the gerbil palatine mucosa were polymorphic, possibly reflecting different function. In order to verify and extend this evidence, the shape of and the innervation to MCs in the palatine mucosa of six different species of rodents including the Mongolian gerbil and the rat were examined by immunohistochemistry and transmission electron microscopy. Immunohistochemistry using anti-cytokeratin 20 (CK20) antibody revealed that in the gerbil palatine mucosa, approximately half of MCs were dendritic. Confocal laser scanning microscopy after double labeling with anti-cytokeratin and anti-PGP 9.5 or anti-Na+/K(+)-ATPase beta 1 subunit antibodies indicated that most of the dendritic MCs (DMCs) in these mucosae were free of innervation. Electron microscopy showed that all species of rodents examined contained abundant dendritic MCs as well as roundish (oval to round) MCs (RMCs) with typical innervation. Secretory granules of the RMCs were usually concentrated at the synaptic site, whereas those of the DMCs tended to accumulate in the tips of the cytoplasmic processes and in the cytoplasm facing the basal lamina. Some MCs showed features intermediate between those of the RMC and DMC. These results indicate that MCs in rodent palatine mucosae are consistently polymorphic, and that DMCs may represent a distinctive subset with specific, presumably including endocrine and paracrine, functions different from those of RMCs.
To elucidate the mechanism of root formation in tooth development, we examined the role of insulin-like growth factor I (IGF-I) on early root formation in mandibular first molar teeth from 5-day-old mice. Immunohistochemistry revealed the specific localization of the IGF-I receptor in Hertwig's epithelial root sheath (HERS) in the tooth root. The effect of IGF-I on root development, especially on HERS, was subsequently examined in vitro. The control culture showed normal development of HERS and the periodontium, resembling that in vivo. However, the presence of 100 ng/ml IGF-I resulted in elongation of HERS and increased cell proliferation in its outer layer. These effects were negated by the addition of antibodies specific for IGF-I. Thus, we propose that IGF-I is involved in early root formation by regulating the mitotic activity in the outer layer of HERS.
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